Solid state lighting based on phosphor downconversion of InGaN LEDs have have begun to replace traditional fluorescent and incandescent lamps. These colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been also produced from Group III-V alloys such as gallium nitride (GaN). Light emitted from InGaN-based LEDs is generally in the UV and/or blue range of the electromagnetic spectrum. Light emitted from the LED is converted to light that is useful for illumination purposes by coating or covering the LED with a phosphor layer. By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored phosphors produce custom colors and higher luminosity, and, in combination with LED generated light, phosphor generated light may be used to produce white light. The most popular white LEDs are based on blue emitting InGaN chips. The blue emitting chips are coated with a phosphor or blend of phosphors that converts some of the blue radiation to a complementary color, e.g. a yellow-green emission. LEDs that emit in the near UV region (405 nm) are coated with a phosphor blend that includes a blue or blue green phosphor and a red emitter. The total of the light from the phosphor and the LED chip provides a color point with corresponding color coordinates (x and y) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
The production of light from LEDs and other light sources typically generates heat as a byproduct. Phosphors exposed to higher temperatures may have decreased quantum efficiency. As the quantum efficiency of different phosphors changes at different rates as temperature increases, the light emitted by the device may dim or the color may shift as the device enters steady state operation. In addition, some phosphors undergo hydrolysis reactions at an appreciable rate under conditions of elevated temperature and humidity. Thus, there is a continued demand for stable phosphor compositions that may be used as components, individually or as part of a phosphor blend, in the manufacture of LED-based lighting systems. Such materials would allow a wider array of light sources with desirable properties including good color quality (CRI>80), a large range of color temperatures, and a relative insensitivity to temperature changes.
Red line emitting Mn+4 doped phosphors have been used in phosphor blends. However, many of these materials exhibit some instability in high temperature, high humidity environments. Therefore, it is desirable to develop new methods to improve the stability of the phosphors.